A Silicon, Carbon and Deuterium NMR Investigation of Molecular Templating in Amorphous Silicas
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ABSTRACT The precise pore sizes defined by crystalline zeolite lattices have led to intensive research on zeolite membranes. Unfortunately zeolites have proven to be extremely difficult to prepare in a defect-free thin film form needed for membrane flux and selectivity. We introduce tetrapropylammonium TPA (a structure directing agent for zeolite ZSM-5) into a silica sol and exploit the development of high solvation stresses to create templated amorphous silicas with pore apertures comparable in size to those of ZSM-5. 29Si and 2H NMR experiments were performed to evaluate the efficacy of our templating approach. The 29 Si NMR spectrum of the silica matrix was observed by an intermolecular cross-polarization experiment between the 'H nuclei of TPA and the 29Si nuclei in the silica matrix. The efficiency of the cross-polarization interaction was used to investigate the degree to which the matrix formed a tight cage surrounding the template molecule. Bulk xerogels, prepared by gelation and slow drying of the corresponding sols, exhibited only weak interactions between the two sets of nuclei. Thin film xeorgels, where drying stresses are greater, resulted in significantly increased interactions. Analogous materials were prepared using fully deuterated TPA. The 2H NMR wideline spectra consisted of a partially narrowed resonance, corresponding to template molecules which were undergoing restricted rotational motion, and an isotropically narrowed resonance, corresponding to molecules which were undergoing rapid rotational motion.
The number of isotropically
rotating template molecules decreased for the thin film specimens, consistent with improved templating of amorphous silica by TPA. INTRODUCTION Zeolite membranes have received considerable attention for gas separation and pervaporation applications [1-3] because of their ordered and uniform pore structure. Zeolites are crystalline materials, however, and difficult to prepare as defect-free thin films needed for separation applications. A molecular templating approach is used to form well-defined porosity in amorphous silica matrices prepared by the sol-gel method. The tetrapropylammonium (TPA) cation, the structure-directing agent in ZSM-5 synthesis, was incorporated as a template within a sol-gel silica matrix. We exploit the development of high solvation stresses to create templated amorphous silicas with pore apertures comparable in size to those of ZSM-5. Organic-inorganic hybrid materials are formed, which on calcination result in TPA-templated microporous silica (Figure 1). The corresponding membranes are ultrathin, have low defect density and show molecular sieving behavior. This paper discusses several approaches based on NMR spectroscopy to characterize the effectiveness of the templating process. Development of a convenient method to measure templating effectiveness will aid in optimizing the synthetic process. 83 Mat. Res. Soc. Symp. Proc. Vol. 576 © 1999 Materials Research Society
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Figure 1. Schematic of TPA templating in sol-gel silica. Organic-ino
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